ECS 36A Lecture Notes - Lecture 20: Call Stack, Compile Time, Time Complexity

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4 Dec 2018
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ECS 36A - Lecture 20 - Ownership, Borrowing, and Lifetime #2
Broken Lifetime Examples:
let x;
{ #start of y’s lifetime
let y = 0:
x = &y #x references y here
} #end of y’s lifetime, x references nothing anymore
println!(“{ }”, x) # x outlives y so this cannot be done
fn f() → &i32 {
let x = 0; #lifetime of x is within the function, x lives on a stack
&x #when the function returns this reference, the def of x doesn’t exist
}
let p = f() # at this point x has disappeared bc stack frame for f disappeared, no reference
FIX for these:
fn f() → &i32 {
let x = 0
x #return the VALUE instead of a reference, or put the reference on a heap
}
let p = f()
Advantage of returning value: avoids lifetime errors
Advantage of returning a reference: avoid a copy of the value, only copy the pointer
Use of pointers: if you have 1 GB of data in f() -- would not want to copy it all, would rather
want to return a pointer for each value
let x = 0;
let y = &x;
let z = &x; # legal to do
# may have multiple shared references to the same valuable, allow you to read only
# multiple readers on the same object have no harm to it
let x = 0; #tells us that x is not mutable, meaning cannot write to x
let y = &mut x; #illegal, mutates x through y
Advantage of immutable variables: allows compiler to do aggressive optimization, reduces
programming errors (cannot accidentally modify it later) → if you have an option, make it
immutable
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